Power semiconductor devices, such as power diodes, power metal-oxide semiconductor field-effect transistors (MOSFETs), power insulated-gate bipolar transistor (IGBTs) or power thyristors, are designed to withstand high blocking voltages. Those power devices include a pn-junction formed between a p-doped semiconductor region and an n-doped semiconductor region. The device blocks (is switched off) when the pn-junction is reverse biased by applying a voltage to the pn-junction. In this case a depletion region or space charge region expands in the p-doped region and the n-doped region. Usually one of these p-doped and n-doped regions is more lightly doped than the other one of these p-doped and n-doped regions, so that the depletion region mainly expands in the more lightly doped region, which mainly supports the voltage applied across the pn-junction. The more lightly doped region supporting the blocking voltage is usually referred to as base region in a diode or thyristor and drift region in a MOSFET or IGBT.
The ability of a pn-junction to support high voltages is limited by the avalanche breakdown phenomenon. As a voltage applied across a pn-junction increases, an electric field in the semiconductor regions forming the pn-junction increases. The electric field results in acceleration of mobile carriers induced by thermal generation in the space charge region. An avalanche breakdown occurs when, due to the electric field, the charge carriers are accelerated such that they create electron-hole pairs by impact ionization. Charge carriers created by impact ionization create new charge carriers, so that there is a multiplication effect. At the onset of avalanche breakdown a significant current flows across the pn-junction in the reverse direction. The electric field at which the avalanche breakdown sets in is referred to as critical electric field. The absolute value of the critical electric field is mainly dependent on the type of semiconductor material used for forming the pn-junction, and is weakly dependent on the doping concentration of the more lightly doped semiconductor region. A voltage blocking capability of the semiconductor device is the voltage applied to the pn-junction at which the critical electric field occurs in the semiconductor device. This voltage is often referred to as breakdown voltage.
The voltage blocking capability is not only dependent on the type of semiconductor material and its doping, but also on the specific geometry of the semiconductor device. A power semiconductor device includes a semiconductor body of finite size that is terminated by edge surfaces in lateral directions of the semiconductor body. In a vertical power semiconductor device, which is a semiconductor device in which the pn-junction mainly extends in a horizontal plane of the semiconductor body, the pn-junction usually does not extend to the edge surface of the semiconductor body. Instead, the pn-junction is distant to the edge surface of the semiconductor body in a lateral direction. In this case, a semiconductor region (edge region) of the semiconductor body adjoining the pn junction in the lateral direction also has to withstand the voltage applied to the pn-junction.
The edge region could be implemented with a planar edge termination structure. In this case, however, the dimension of the edge region in the lateral direction of the semiconductor body is usually a least between two times and three times the dimension (length) of the drift region (base region) in the vertical direction. The length of the drift region (base region) is dependent on the desired voltage blocking capability of the device and can be up to several 10 micrometers (□m), so that a corresponding edge termination would be very space consuming.
In order to reduce the space required for withstanding the blocking voltage in the edge region, a vertical edge termination, which is also referred to as mesa edge termination, can be provided. Such vertical edge termination includes a trench filled with a dielectric material. Suitable dielectric materials such as benzocyclobutene (BCB) based dielectrics, to name only one, have a high dielectric strength of several MV/cm and a low leakage current. However, the leakage current may increase as the electric field strength increases. In a mesa edge termination, the electric field strength can be particularly high at an interface between the semiconductor material surrounding the trench and the dielectric material, so that increased leakage currents may occur in these regions.
There is therefore a need for an improved edge termination for semiconductor devices, in particular semiconductor devices having a semiconductor body with a rectangular geometry.